Automotive instrument clusters often use a vacuum fluorescent (VF) tube which provides an illuminated display. The filaments of typical VF tubes used in the automotive environment are driven by approximately 1.5 to 1.8 volts DC. This voltage is obtained by taking the battery voltage (approximately 13.2 volts) and reducing it to the required voltage through the use of dropping resistors. The normal current of the filament is 300 to 400 mA. thus, the resistors used are large wattage and generate a lot of heat. This is not desirable or practical for many applications due to cost and packaging constraints.
The operation of a VF tube only requires that the filament be heated to a certain level by application of current. By providing a pulsed current of larger amplitude than the DC voltage normally used to drive the VF filament, the high wattage dropping resistors can be eliminated. Even battery voltage may be used. The duty cycle of the pulses is chosen to provide the necessary power level of the filament, and the frequency is above audible range to prevent audible noise or singing of the filament. However, when pulsing the VF filament in this manner the radiated emission level may be objectionable.